1. Field of the Invention
The present invention generally relates to the field of optical fibers, in particular a fiber optical cable, ribbon, fiber or buffer tube having a gel-swellable layer for added protection and stability of the fibers in the cable structure.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable (including wire cable) networks. The use of optical fibers in today's technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Many of the areas of use for optical fibers, such as communications, require the optical fibers be protected from various destructive factors, such as adverse weather, moisture, impact damage, etc. This protection for the fibers can come in various ways.
One common way to protect fibers, or fiber ribbons, is to have a water resistant gel placed between the individual fibers, or fiber ribbons, and the cable buffer tubes, or the fiber ribbon stacks and the buffer tubes. This water resistant gel minimizes the amount of water or moisture that enters the fiber optic cable and protects the fibers from impact damage during installation, storage, repair, etc. An example of such a gel, along with its characteristics can be found in U.S. Pat. No. 6,085,009 to Risch et al., which is incorporated herein by reference.
One requirement for fiber optic cables is that the waterblocking gel within a fiber-optic cable does not exude out the end of the cable, even at elevated temperatures. Satisfactory resistance to flow of the waterblocking gel may be problematic especially for large diameter cables at high temperatures. Attenuation in individual fibers (loss of signal power in the fibers) may also be a problem in large cables containing fiber ribbons. In this case attenuation is generally attributable to microbending in the fibers, which tends to occur when the outer most fibers make contact with the walls of the buffer tubes or outer jackets. This is most common in the outer most fibers (corners) of ribbon stacks.
This is particularly true in fiber optic cables having large diameter (larger than 0.310″) buffer tubes where the cross-sectional area of the gel layer becomes so large that the gel is not stable, and will not provide adequate damage or moisture protection. Therefore, the use of only a gel layer between the fibers and buffer tube (or outer jacket) of a cable limits the overall size the cable can be, to ensure adequate protection of the fiber(s).
One common test for determining whether or not a protective gel material is adequate, is having the sample cable (with the gel compound) cut and vertically positioned in an oven. The temperature in the oven is then raised to approximately 70° C. for a period of 24 hours. As the temperature of the compound or gel increases the viscosity and yield stress of the compound or gel decreases. When the viscosity and shear stress decreases, to a certain point, the gel or compound will drip or leak from the cut portion of the cable. This dripping or leaking is evidence that the particular gel or compound is not acceptable or the tube size is too large compared to a relatively small fiber bundle or ribbon stack for applications in fiber optic cables where such conditions may exist (i.e. higher temperatures).
In general, the compound or gel flow is governed by a number of different factors, such as: gravity, yield stress of the material, oil separation in the material, viscosity of the materials, coefficient of expansion of the material, adhesion or slippage of the material or compound to the walls of the buffer tube or outer jacket, and the diameter (cross-sectional area of the gel) of the cable.